This invention relates to machine tools and more particularly to an assembly for holding workpieces down on worktables of machine tools.
In one type of machine tool used extensively in the woodworking, plastics and aerospace industries, there is provided a base member having a table on which a workpiece may be supported, a bridge or gantry member supported on the base member and adapted to be displaced relative to the base member along a longitudinal line of travel or what is commonly referred to as an x-axis, a toolhead support assembly supported on the bridge member and adapted to be displaced relative to the bridge member along a transverse line of travel or what is commonly referred to as the y-axis, and a toolhead assembly having one or more tools mounted thereon and engageable with a workpiece positioned on the table, supported on the toolhead support assembly and adapted to be displaced along a vertical line of travel or what is commonly referred to as the z-axis. In a modified form of such machines, the table portion of the machine is mounted on the base member for displacement relative to the base member along the x-axis, and the bridge is stationary so that workpieces positioned on the table are displaced along the x-axis into contact with a tool of the toolhead assembly which is adapted to be displaced relative to the stationary bridge member along the y and z-axes.
In either of such types of machines, it is necessary to position such workpieces on the worktable and further to hold them down on the worktable in order to perform the machining function. Typically, positioning means such as pop-up pins have been used to accurately position the workpieces on the worktable and various holddown means such as clamps, vacuum systems and the like have been used to secure the workpieces to the worktables.
In applications where stacks of workpieces are to be machined, and it has been either impractical to use clamping devices or not possible to use vacuum systems for holding down the stack of workpieces, it has been a common practice to use what has been commonly referred to as roller holddown assemblies. Such assemblies commonly have consisted of a set of brackets mounted on a displaceable or stationary bridge or gantry member of the machine, sets of air cylinder assemblies each having their base members rigidly secured on such brackets with upwardly extending rod members, and transversely disposed rollers engageable in rolling contact with an upper one of a stack of workpieces positioned on the worktable, having end portions thereof journaled in bearings provided in the upper ends of the rod members of the air cylinder assemblies. In the use of such holddown assemblies, the rollers are first positioned over a stack of workpieces mounted on the worktable and then the air cylinders are operated to retract the rods thereof and cause the rollers to engage the uppermost one of the stack of workpieces in rolling contact therewith. As the bridge member is displaced relative to a stationary worktable or a worktable is displaced relative to a stationary bridge member, along the z-axis, such rollers will apply a downward force on the stack of workpieces while permitting the workpieces to be displaced relative to the rollers.
Although such holddown assemblies have been effective in holding down a stack of workpieces being machined, it has been found that in the use of such assemblies, the downward force applied by the rollers is transmitted downwardly through the bearings of the tables or bridge members thereof thereby placing undue loads on such bearings resulting in undue wear or premature failure. It thus has been found desirable to provide a holddown assembly generally of the type described which obviates the problem of placing undue loads on the bearings of the tables or bridge members thereof.
Accordingly, it is the principal object of the present invention to provide an improved means for holding down workpieces on a worktable of a machine tool.
Another object of the present invention is to provide an improved means for holding down a stack of workpieces on a worktable of a machine tool.
A further object of the present invention is to provide an improved means for holding down workpieces on a machine tool having either a moveable table or a moveable bridge or gantry member.
A still further object of the present invention is to provide an improved means for holding down workpieces on the table of a machine having a moveable table or a moveable bridge or gantry supporting a toolhead assembly of the type utilizing a set of rollers engaging an uppermost one of a stack of workpieces in rolling contact therewith.
Another object of the present invention is to provide an improved assembly for holding down a stack of workpieces on the table of a machine provided with either a moveable table or a moveable bridge or gantry member in which the application of undue loads on the bearings of such tables or bridge or gantry members is avoided.
A further object of the present invention is to provide a roller type of workpiece holddown assembly for a machine tool utilizing a moveable table or a moveable bridge or gantry member which is simple in design, comparatively inexpensive to manufacture and service and highly effective in performance.
Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings wherein: